The present invention relates to a novel process for the preparation of 2,3-pentanedione, wherein the process is derived from hydroxyacetone and paraldehyde.
2,3-Pentanedione is an important aroma substance (Allured""s Flavor and Fragrance Materialsxe2x80x941995, Allured Publishing Corporation, 1995, p. 22). In addition, 2,3-pentanedione is an important raw material for the preparation of alkyl-substituted pyrazines (DE 100 22 361 C1).
The preparation of 2,3-pentanedione is known per se. The preparation takes place, for example, starting with hydroxyacetone and paraldehyde in a single-stage process and can be illustrated by the following scheme: 
The known synthesis process, as disclosed in U.S. Pat. No. 2,799,707 produces the desired 2,3-pentanedione from hydroxyacetone and paraldehyde in an uneconomic process. The poor yield of 48% of 2,3-pentanedione and the long reaction time (about 60 hours) are particularly disadvantageous.
A process, which produces 2,3-pentanedione in a simple manner and in good yield, is therefore of great interest.
The present invention therefore provides a process for the preparation of 2,3-pentanedione from hydroxyacetone and paraldehyde, wherein the reaction is carried out in the presence of an aqueous phase containing a strong organic or inorganic acid with a pKa value of less than or equal to 4 and in the presence of a phase transfer catalyst.
It has been found that the addition of a phase transfer catalyst can improve the yield compared with the prior art (See for example, U.S. Pat. No. 2,799,707). In addition, the reaction time can be shortened and the space-time yield increased.
Preference is given to the addition of a metering mixture containing hydroxyacetone and paraldehyde and optionally a diluent to an aqueous solution containing an acid with a pKa value of less than or equal to 4 and a phase transfer catalyst.
Suitable phase transfer catalysts for the process according to the present invention include, for example, phosphonium, sulfonium and ammonium compounds. Preference is given to ammonium compounds. Preferred ammonium compounds correspond to the formula
R1R2R3R4N+Xxe2x88x92
where
R1 to R4xe2x80x94independently of one anotherxe2x80x94are C-1 to C-18-alkyl or benzyl and
Xxe2x88x92 is iodide, chloride, hydroxide or hydrogen sulfate.
More preferred phase transfer catalysts are the tetrabutylammonium salts (i.e., wherein R1 to R4 are butyl).
Acids useful in the present invention with a pKa value which is less than or equal to 4 may be inorganic and organic acids. Acids with a pKa value of less than or equal to 1 are preferred. More Preference is given to inorganic acids. Most preference is given to HCl, HBr, sulfuric acid and nitric acid.
The amount of acid is 5 to 50% by weight, based on the amount of aqueous phase, preferably 10 to 30% by weight, based on the amount of aqueous phase.
Typically, the amount of paraldehyde, based on hydroxyacetone used, is 40 to 200% by weight, preferably 50 to 100% by weight.
Typically, the amount of paraldehyde in the metering mixture is 40 to 200% by weight, based on the amount of hydroxyacetone, preferably 50 to 100% by weight.
The metered addition of the metering mixture takes place at a reaction temperature (bottoms temperature) in the range from 35 to 100xc2x0 C., preferably 40 to 75xc2x0 C., more preferably 45 to 60xc2x0 C. The temperature of the metering mixture is usually in the range from 0 to 40xc2x0 C., preferably in the range from 10 to 30xc2x0 C. The metering time is usually 1 to 30 hours, preferably 5 to 20 hours.
The process according to the present invention can be carried out in the presence of diluents. The addition of inert organic diluents is not required. A preferred diluent is water.
When the metered addition of the metering mixture is complete, a post reaction time may follow, preferably a post reaction time in the range from 0.05 to 3 hours, more preferably in the range from 0.1 to 1 hour.
Preferably, the completion of the metered addition is followed directly by distillation of the mixture containing water and 2,3-pentanedione. This distillation can be carried out under atmospheric pressure or under reduced pressure. In addition, this distillation can also be carried out with superheated steam, which is introduced into the reactor. The steam pressure is in the range from 3 to 10 bar.
Preference is given to a steam distillation at atmospheric pressure by the addition of water.
The chemical yield is typically around 85% of the starting material used in deficit, as a result of which an economic preparation of 2,3-pentanedione is possible.
According to the process of the present invention, it is possible to prepare 2,3-pentanedione with good reaction yield under gentle reaction conditions without the use of complex apparatuses from commercially available starting materials. The process according to the present invention is therefore suitable for the economic preparation of 2,3-pentanedione on a pilot-plant and industrial scale.
The process according to the present invention can, for example, be carried out as follows:
A metering mixture containing hydroxyacetone and paraldehyde and optionally a diluent is added dropwise to an aqueous solution containing an acid with a pKa value of less than or equal to 4 and a phase transfer catalyst. The reaction temperature is kept relatively constant, and is in the range from 35 to 100xc2x0 C.
When the metered addition is complete, and optionally after a post reaction time, water is added, and a mixture which contains water and 2,3-pentanedione is distilled off from the reaction mixture. When distillation is complete, the 2,3-pentanedione is separated off from the water. The purity of the 2,3-pentanedione produced is 85 to 95% by weight. If required, further purification may take place, for example, by further distillation.